The present invention relates to a cathode ray tube apparatus, or more in particular, to a cathode ray tube apparatus comprising a deflection yoke capable of reducing the deflection power and the leakage magnetic field effectively.
Generally, the cathode ray tube apparatus comprises a vacuum envelope of glass and a deflection yoke forming a deflection magnetic field for deflecting the electron beams. The vacuum envelope includes a rectangular faceplate, a cylindrical neck portion and a funnel portion for coupling the faceplate and the neck portion to each other. The deflection yoke is mounted over the portion extending from the neck portion to the yoke portion in the funnel portion.
In the cathode ray tube apparatus having this construction, the deflection power supplied to the deflection yoke is the main power consumed in the apparatus. In recent years, in order to satisfy the requirement for high brightness and high definition of the cathode ray tube apparatus, the trend is toward an even more increased deflection power. For the power consumption of the cathode ray tube apparatus to be reduced, however, the deflection power is required to be decreased.
Generally, for reducing the deflection power, the outer diameters of the neck portion and the yoke portion are desirably reduced. With this structure, the operating space of the deflection magnetic field is reduced and the operating efficiency of the deflection magnetic field exerted on the electron beams is improved.
In the conventional cathode ray tube apparatus, however, if the outer diameters of the neck portion and the yoke portion are reduced, the electron beam having a large deflection angle, that is, having an electron beam trajectory at a large angle to the tube axis impinges on the inner wall of the yoke portion. Such an electron beam fails to reach the phosphor screen and causes a display failure. In the cathode ray tube apparatus having this construction, it is difficult to reduce the deflection power and the leakage magnetic field by reducing the outer diameters of the neck portion and the yoke portion.
U.S. Pat. No. 3,731,129 discloses a cathode ray tube in which the shape of a section perpendicular to the tube axis of the yoke portion changes progressively from a circle to a rectangle starting with the neck portion toward the faceplate in approximation with the passage area of the electron beam. With this pyramidal yoke portion, the outer diameter of the yoke portion can be reduced while preventing the electron beam from impinging on the inner wall of the yoke portion. Also, with this structure, the deflection magnetic field acts on the electron beam with a comparatively high efficiency.
In the cathode ray tube apparatus of this configuration, however, the side surfaces of the yoke portion flatten more and the environmental pressure resistance of the yoke portion of the vacuum envelope is reduced more, the higher the rectangularity of the sectional shape of the yoke portion. Thus the safety is adversely affected.
Recently, a flat display unit with a flat outer surface of the faceplate has found an application. In the flat display unit with an outer surface having a radius of curvature at least twice the effective diagonal length of the phosphor screen (the faceplate is completely flat when the radius of curvature is infinitely large), however, the environmental pressure resistance of the faceplate is low and the yoke portion, if pyramidal, decreases also in the environmental pressure resistance, thereby making it difficult to secure a mechanical strength required of the vacuum envelope as a whole for safety. The strength of the vacuum envelope, that is, the environmental pressure resistance and the mechanical strength thereof combined will hereinafter be collectively called the bulb strength.
FIG. 10A shows a structure in which a conical deflection yoke 105 is mounted on a yoke portion 103 of the cathode ray tube disclosed in U.S. Pat. No. 3,731,129. FIG. 10B shows a horizontal deflection magnetic field distribution MF1 formed by the deflection yoke 105. In this deflection magnetic field distribution MF1, a position of the deflection center where the deflection magnetic field is most concentrated is called C.
As shown in FIG. 10C, in the case where the yoke portion 201 is formed substantially pyramidal and the deflection yoke 202 is formed in conformance with the outline of the yoke portion 201, the distance D2 between the inner surface at the end portion 203a of the horizontal deflection coil 203 nearer to the screen and the tube axis Z is smaller than the distance D1 of the horizontal deflection coil 110 shown in FIG. 10A, and therefore the electron beams become nearer. In this case, the position of the deflection center of the horizontal deflection magnetic field distribution MF2 shifts toward the screen from point C, as shown in FIG. 10D.
FIG. 11 is a diagram for explaining the trilemma indicating the degree of misconvergence. In the case where rectangular rasters 204 and 205 are plotted by a pair of side beams on the phosphor screen, the trilemma Tr is expressed as XH-YH+PQV. In the case where the rasters 204 and 205 completely coincide with each other, that is, in the case where Tr=0, it is assumed to be a reference state. As shown in FIGS. 10C and 10D, when the position of the deflection center of the horizontal deflection magnetic field distribution MF2 shifts toward the screen from point C, the trilemma Tr increases in positive direction from the reference state and deteriorates the convergence characteristic.
A method of correcting the deterioration of the convergence characteristic may be to move the end portion 203a of the horizontal deflection coil 203 nearer to the screen back toward the neck along the tube axis Z. This method, however, has the disadvantage of reducing the deflection sensitivity.
Another method may be to extend the end portion of the horizontal deflection coil 203 nearer to the neck toward the neck along the tube axis Z. The horizontal deflection coil 203 shown in FIG. 10C has what is called a bend-up type of end portion, in which, with the increase in the number of turns, the crossover portion 206 near to the neck is wound progressively in the direction away from the tube axis Z, that is, in radial direction of a circle having the tube axis as a center axis. With the horizontal deflection coil 203 having this structure, the magnetic field leaking toward the neck is strong. As a result, the end portion of the horizontal deflection coil 203 is required to be extended more toward the neck along the tube axis Z. This is structurally difficult to realize.
In the conventional cathode ray tube apparatus, the two requirements described above, that is, a rectangular section of the yoke portion in order to sufficiently reduce the deflection power and a sufficient bulb strength even with a rectangular section of the yoke portion on the other, cannot be met at the same time. It is especially difficult for the cathode ray tube apparatus for flat display to reduce the deflection power and to secure a sufficient bulb strength at the same time.
Also, in the conventional cathode ray tube apparatus, if the deflection yoke is used which simply conforms with the outline of the substantially pyramidal yoke portion without taking the magnetic field leaking toward the neck into consideration, the problem of a deteriorated convergence is posed.